The selective chemical vapour deposition (CVD.) of tungsten is used for providing the interconnection of the metal layers during the manufacture of electronic circuits. The principle is as follows: the substrate to be treated includes a conducting layer, which is either a metal layer (tungsten, aluminium, titanium or their derivatives) or a silicon layer, covered with a silicon-oxide layer in which holes passing right through it are made at the desired locations. These holes are intended to be filled with tungsten, which thus makes it possible to provide the connection between conducting layers on either side of the silicon-oxide layer.
It is known that high deposition temperatures (between 330.degree. and 400.degree. C.) considerably improve the performance of the tungsten deposition, making it possible in particular to increase the adhesion (no disbonding, decreased contact resistance) and to decrease the stress (less than 1 gigapascal) and the resistivity of the tungsten deposited.
However, the main problem posed by the selective deposition of tungsten, and in particular at high temperatures, relates to the effective selectivity of the deposition. This is because, ideally, the tungsten should be deposited solely inside the holes made in the silicon oxide, in contact with the metal appearing at the bottom of these holes and not on the silicon-oxide layer itself. In practice, this result is very difficult to obtain, and this loss-of-selectivity problem increases as the deposition temperature increases. Generally it is considered impossible to carry out the selective vapour deposition of tungsten at temperatures above 300.degree. C.; for example, K. Y. AHN et al., [Tungsten and other refractory metals for VLSI, Application IV MRS (1989)], have studied the growth of the selective tungsten deposition and have shown the loss of selectivity at temperatures above 300.degree. C.
It would therefore be particularly desirable to manage to develop a method enabling tungsten to be deposited at high temperatures without a loss of selectivity resulting.
Various teams have studied the factors affecting the deposition of tungsten onto the silicon oxide, and the effects of various treatments on this deposition.
It is known (cf., for example, Application EP 0,430,303 in the name of APPLIED MATERIALS INC) to carry out, prior to the tungsten deposition, a cleaning pretreatment in order to descale the substrate, and to remove the moisture and the surface layers of oxides which interfere with the tungsten deposition; the nature of this treatment depends on that of the surface onto which it is desired subsequently to deposit tungsten selectively. For example, according to the Application EP 0,430,303, a treatment by a halogen-containing gas, such as BCl.sub.3, for the aluminium and the aluminium oxides, a treatment by NF.sub.3 or SF.sub.6 for the silicon and its oxides and a treatment by hydrogen for the tungsten will preferably be chosen. The pretreatment is performed in a cleaning chamber and the substrate is subsequently transferred into a deposition chamber with a view to a selective deposition of tungsten.
KAJIYANA et al. [Mat. Res. Soc. Symp. Proc. VLSIV, p. 39 (1990)] describe the effect of a pretreatment, by a CF.sub.4 +CHF.sub.3 plasma followed by a NF.sub.3 treatment, on the deposition of tungsten onto a silicon substrate. The purpose of this treatment is to prevent the formation of an irregular silicon-oxide layer at the surface of the substrate, since the presence of this layer subsequently produces irregularities in the deposition of tungsten. The CF.sub.4 +CHF.sub.3 plasma treatment has the effect of implanting a layer of carbon atoms at the surface of the silicon. These carbon atoms are subsequently themselves removed by sweeping the surface with a NF.sub.3 plasma. The quality of the tungsten layer deposited onto a substrate thus treated has been determined by comparing a substrate carrying a native silicon-oxide layer with a substrate pretreated by immersion in a solution of hydrofluoric acid. This has made it possible to show that the removal of the oxide layer followed by a treatment by NF.sub.3 enables the tungsten-deposition temperatures to be lowered; the film formed at 280.degree. C. is similar, from its homogeneity and its regularity, to that obtained when tungsten is deposited at 300.degree. C. after treatment of the substrate by immersion in hydrofluoric acid. The pretreatment described by KAJIYANA et al. therefore has the effect not of increasing the selectivity of the deposition at high temperature but of improving the characteristics of the deposition performed at low temperature.
TAMARU et al., [Dry Process Symposium, Electrochemical Society, Pennington N.J. (1990)] have studied the effect of various surface treatments on the selectivity of vapour deposition of tungsten onto the surface of the tungsten appearing at the bottom of holes made in a silicon-oxide layer. It emerges from this study that it is the BCl.sub.3 pretreatment which enables the selectivity of the tungsten deposition to be increased most effectively. According to these authors, this treatment generates Si--Cl bonds as well as SiCl.sub.x and SiCl.sub.x O.sub.y molecules which remain for a sufficient time at the surface of the silicon-oxide layer in order to protect it from being etched by tungsten hexafluoride molecules during the deposition of tungsten by CVD. These same authors also indicate that a NF.sub.3 treatment does not enable a comparable selectivity to be obtained, this being due to the fact that the Si-F bonds provide only weak protection of the silicon oxide from being etched by the WF.sub.6.
Now, the inventor has now developed a method enabling the selectivity of the vapour deposition of tungsten to be improved and excellent selectivity to be obtained at deposition temperatures above 300.degree. C.